1. Field of the Invention
The present invention relates to a conductive masterbatch. More particularly, the present invention is concerned with a conductive masterbatch comprising a polyamide and conductive carbon black, the conductive carbon black being present in the form of at least one agglomerated particle having a major axis of 20 to 100 μm, wherein the number of the agglomerated particle(s) is 1 to 100 as observed under an optical microscope with respect to a contiguous area of 3 mm2. By the use of the conductive masterbatch of the present invention, it becomes possible to obtain a conductive resin composition which not only has excellent heat resistance, but also simultaneously exhibits excellent conductivity and impact resistance (wherein the “excellent conductivity” means a conductivity which is comparable or superior to that of a conventional material for use in a processing involving an electrostatic coating, i.e., conductivity sufficient for a material to be electrostatically coated). Such a conductive resin composition can be used in a wide variety of fields, such as electric and electronic parts, parts of office automation machines, automobile parts and other mechanical parts. Especially, the conductive resin composition is very advantageous as a material for producing an automobile outer panel (e.g., an automobile fender) because when the above-mentioned resin composition is molded into a large article (such as an automobile outer panel, a door panel or the like) and the resultant molded article is then subjected to an electrostatic coating, an excellent molded article can be obtained, wherein the molded article is unlikely to suffer not only heat distortion but also a lowering of the impact resistance and wherein an excellent coating can be formed on such a molded article due to the excellent conductivity of the resin composition. The present invention is also concerned with a method for efficiently producing the above-mentioned resin composition.
2. Prior Art
Polyphenylene ethers not only have excellent mechanical and electrical properties and excellent heat resistance, but also have excellent dimensional stability. Therefore, polypheneylene ethers have been used in a wide variety of fields. However, the moldability of a polyphenylene ether is poor. For improving the moldability of a polyphenylene ether, Examined Japanese Patent Publication No. Sho 45-997 discloses a technique in which a polyamide is added to a polyphenylene ether, to thereby obtain a polyamide-polyphenylene ether alloy. Nowadays, polyamide-polyether alloys are used in a very wide variety of fields.
Recently, as an electrostatically coatable material, use of a conductivity-imparted polyamide-polyphenylene ether alloy is rapidly expanding in the fields of automobile outer panels (such as a fender and a door panel). For example, it has been attempted to use an automobile fender made of a polyamide-polyphenylene ether alloy for the purpose of improving not only the safety of automobiles (e.g., protection of pedestrians) but also the ability of an automobile fender to recover from distortion.
The materials used for an automobile outer panel are required to have various good properties, such as a conductivity sufficient for electrostatic coating, impact resistance, heat resistance and melt-fluidity.
With respect to the method for imparting a conductivity to a polyamide-polyphenylene ether alloy, for example, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 2-201811 discloses a technique to lower the surface resistance of a polyamide-polyphenylene ether alloy by a method in which carbon black is caused to be contained mainly in the polyamide phase of the polyamide-polyphenylene ether alloy, or by a method in which carbon black is uniformly dispersed in a polyamide, followed by mixing thereof with a polyphenylene ether. There have generally been known a method for uniformly dispersing fine particles (such as particles of carbon black) in a high viscosity substance, and products obtained by such a method (see, for example, “Kagakugijutsushi (Journal of Chemical technology) MOL”, pp. 41–46, published by Ohmsha Ltd. Japan, November 1987.
Unexamined Japanese Patent Application Laid-Open Specification No. Hei 8-48869 (corresponding to U.S. Pat. No. 5,977,240) discloses a technique in which a polyamide and a polyphenylene ether are compatibilized with each other in advance, followed by addition of carbon black, to thereby obtain a resin composition having good impact strength, good melt-fluidity, and low volume resistivity.
Further, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 4-300956 (corresponding to EP 506386) describes that, in the production of a composition containing a polyamide, a polyphenylene ether, carbon black and a compatibility agent, by adjusting the amounts of these components and using a polyphenylene ether and a polyamide which have specific relative viscosities, it becomes possible to improve the conductivity and processability of the composition.
International Patent Application Publication No. WO 01/81473 discloses a technique in which a conductive carbonaceous filler (Ketjen Black (KB)) is caused to be present in a particulate form in the polyphenylene ether phase of a polyamide/polyphenylene ether resin composition. In this patent document, the structure of the resin composition is observed under a transmission electron microscope with a magnification as high as 20,000, but the observation was made only to confirm the presence of KB particles (size: around 50 nm) which are much smaller than 20 μm. Further, the impact resistance of the resin composition was unsatisfactory. In this patent document, the resin composition was produced using a masterbatch; however, there is no description about the specific features of the masterbatch.
The resin compositions and molded articles obtained by the above-mentioned conventional techniques are likely to suffer heat distortion. Further, conventionally, it has been impossible to produce a resin composition which is improved with respect to all of the conductivity, impact resistance and heat distortion resistance. Particularly, the conventional resin compositions were not satisfactory as a material for producing an article (such as an automobile outer panel) which is large in size and which needs to be used in a processing involving an electrostatic coating and needs to have high heat distortion resistance, high impact resistance and conductivity sufficient for electrostatic coating. Therefore, the development of a new technique to produce a material suitable for use in the production of the above-mentioned article has been desired in the art.